DE4127482A1 - SHUT-OFF SYSTEM IN A TELEVISION RECEIVER - Google Patents

Description

The present invention relates to a protective circuit for a load, in particular a protective circuit for a DC current load associated with the output of an AB or B push-pull ver is connected by a "secondary tension ungs "power supply (" split voltage "power supply) is fed.

Conventional amplifiers used in audio systems that don't use output transformers are often in shape a push-pull amplifier configuration with paired transis interfere in the output stage, which has an on cell voltage power supply are connected in series, which is more common is referred to as a "totem pole" configuration. In egg In such a case, the output signal becomes a speaker fed between the connection of the output trans storen pair and mass over a relatively large coupling capacitor is connected, the consumer goods devices Capacitor is usually an electrolytic capacitor. A such an output coupling capacitor prevents the DC voltage at the connection of the output transistors, which is at about half the power supply voltage above ground is fed to the speaker. However, you can low-frequency audio signals through the coupling capacitor like bass signals attenuated considerably and therefore not from the sound speakers are played. In addition, such a ger coupling capacitor a so-called "unpoled" Elek trolytic capacitor so that the high-frequency signals not by the condenser's inner coil winding inductive reactance caused by door plates the. Such an "unpoled" electrolytic capacitor is included comparable capacitance and voltage rating Lich more expensive and larger than a standard "poled" Electrolytic capacitor.  

An alternative approach that discusses the above disadvantages with respect to an output coupling condenser avoids, is a "secondary voltage" network some with the same plus and minus power supply unit in terms of mass to use voltages. In this case, the output transistors with each other across the entire power supply the connected in series. This configuration al lerdings that the bias for the output transistors is set so that the connection point of the Transi interfere essentially with DC ground potential will. The speaker can therefore directly between the Ver connection point of the transistors and ground are switched, without the need for an output coupling capacitor since both sides of the speaker are essentially on the same DC potential from ground, so that No DC voltage is supplied to the loudspeaker. Hence the coupling capacitor and audio signals can be omitted with very low bass frequencies, the speaker can be practical table are fed undamped. The latter property is particularly important for audio systems with higher lei stung, which usually have larger speakers, wel che for improved reproduction of the bass-frequency signals are able.

A risk of such a direct coupling of the loudspeaker chers to the connection of the amplifier output transistors is that there is a DC voltage at the output terminal and can be present at the loudspeaker when in the amplifier Error occurs. In a common "single voltage" network sub-system, the output coupling capacitor holds a sol Chen, DC voltage-generating errors from the speaker unless the coupling capacitor itself fails. In any case, such an output DC voltage damage the speakers.  

Some known amplifiers see either between the Output transistors and the power supply or in series with the Speakers or both, fuses to protect against increased currents in the speakers due to DC voltage before. However, it is not easy to make fuses Send dimension because they are also exposed to signal currents are and can "burn out" even if there is no error condition is present. They also work quickly responding backups often not with sufficient Ge speed to protect the output transistors from damage to protect.

Another approach is to use a "DC Detect gate "circuit to use one from one to the sound outgoing error to speaker applied DC know and turn off the amplifier before doing any damage entry. Such a detector is shown in U.S. Patent No. 40 10 402 by Miyata. Such a circuit provides that a Low pass filter for low frequencies has a threshold de tector for actuating a relay or silicon-controlled Rectifier (SCR) controls to turn off the speaker switch or short-circuit.

TV receivers are often with different security devices equipped. When a safety-related A number of measures can be taken in an impending state will. If a microprocessor is used, the Mikropro be programmed to be the primary Turns off power and a predetermined number of Paint tries to turn the receiver back on. Will after the given number of attempts of the error still detected, the microprocessor keeps the receiver in abge switched state. Alternatively, a "fuse" scarf If an error is detected, only switch the device "off" and let it "out". The latter type of circuit takes place  in the TX-81 chassis from Thomson S.A., France, Verwen dung. More specifically, the chassis is with an x-ray len protection circuit provided when an error is detected in a capacitor in the vertical deflection stage, where a sol error could cause damage if it persisted de, is triggered to the horizontal deflection circuit, and especially the horizontal output transistor ten. This shutdown of the horizontal deflection circuit a secondary power supply fed by the line end transformer delivery. In the chassis TX-81 supplies the secondary power Power supply at least to the vertical deflection stage and the audio part. Such a circuit for the TX-81 chassis is disclosed in the RCA patent application number 85 758, filed det as "British Provisional Application" (provisional briti registration) No. 89 29 103.3.

The invention relates to a shutdown system, which on the Detect an error in an audio power amplifier of a television system and which particularly protection exploits circuits to prevent damage due to other errors. In the preferred Embodiment is that from the audio amplifier to the loudspeaker ver applied voltage with a DC detector bound where they are low-pass filtered and a threshold detection gate is fed. The normal audio signal is attenuated, so that the detector doesn't fire, but persistently that Direct current passing through the filter triggers the detector. The Output of the detector is with the x-ray protection circuit of the receiver connected to the receiver "turns off" when an audio error is detected.

The invention is explained in more detail with reference to drawings. Show it:  

An illustration in block diagram form of a conventional television receiver in which the present invention can be attached Wandt Fig. 1,

Fig. 2 is an illustration in block diagram form a Anord voltage in accordance with aspects of the present invention, and

Fig. 3 is a schematic of the low pass filter, the error detector and the trigger for switching off the arrangement in Fig. 2 according to certain other aspects of the present inven tion.

Fig. 1 shows a conventional television receiver arrangement. An RF television signal supplied by a source such as an antenna, a cable system, a satellite receiver, a video recorder or the like is received at the input terminal 12 and switched to the tuner 14 , in which the received signal by means of a mixer and a receiving oscillator (not shown) is selectably selected and superimposed in order to generate an IF signal at output 16 . The IF signal is switched to the IF processor and detector 18 , which generates a baseband video signal which is connected to the video processor 22 via the output 20 , and an audio signal which is connected to the audio processor 26 via the output 24 . Video processor 22 includes luminance, chrominance, and synchronous isolation (synchronous) conditioning circuits and generates color video signals that are switched to CRT (CRT) 30 outputs 28 . The horizontal (H) and vertical (V) synchronous signals generated by the video processor 22 are connected to the deflection and high voltage part 23 , which generates horizontal (H) and vertical (V) deflection signals and end anode supply voltages for CRT 30 . Audio processor 26 detects, amplifies and matrices the audio component contained in the audio signal detected within part 18 in order to make 29 , 31 audio signals available for right and left channel, for amplification and processing by corresponding audio channel processors 32 , 34 , which corresponding power amplifiers 48 , 50 include, as shown in FIG. 2. The signals from processors 32 , 34 are connected via corresponding outputs 36 , 38 to corresponding speakers 40 , 42 . Power supply unit 44 it receives current from a suitable energy source, such as an AC power source or battery (not shown) and generates various power supply unit supply voltages for supplying the various components of the device described above, symbolically indicated as 46 terminals. A microprocessor-based controller 62 controls various parts of the television receiver, such as tuner 14 and power supply 44 .

Deflection and high voltage stage 23 , which is included in video stage 29 , contains an X-ray protection circuit (not shown in FIG. 1), as is usually found in television receivers, for protection against possibly security-threatening conditions which can occur in the receiver, e.g. B. excessive tube beam currents or excessive tube renendanodenspannungen which can result in the generation of Röntgenstrahl len above approved safety regulations when the electrons of the scanning beam hit the shadow mask and the faceplate. A common procedure when detecting such a condition is to switch off the horizontal deflection circuits which supply the current for the row end circuit from which the end anode voltage is derived. Alternatively, as shown in Fig. 1, the power supply can be switched off, whereby the entire receiver including the horizontal deflection circuits is switched off. With the use of microprocessors, selective measures can be taken, such as B. Turning off only certain selective circuits or only certain parts of the main power supply or secondary power supplies A suitable x-ray protection circuit is disclosed in US Patent No. 46 41 064 issued to Testin et al. for a "switch-off circuit for television power supplies".

As shown in Fig. 1, the X-ray protection circuit of the deflection and high-voltage part 23 , in response to the detection of a state which may result in the generation of X-rays, a corresponding signal, referred to in Fig. 1 as XRP . The XRP signal is connected to microprocessor controller 62 , which in the exemplary embodiment is programmed to switch an "off" command to power supply 44 . As a result, the receiver is turned "off" and the generation of potentially harmful X-rays is prevented.

In the television receiver shown in FIG. 1, the X-ray protection arrangement is advantageously additionally used to protect right and left loudspeakers 40 , 42 from DC faults that occur in connection with the failure of power amplifiers 48 , 50 within the line and right audio signal channel processors 32 and 34 occur. In detail, the output signals (L DC FAULT and R DC FAULT) of the DC detectors within the corresponding processors 32 and 34, respectively, using a - symbolically represented by "OR" gate 63 - "OR" func on circuit with one input connected by microprocessor controller 62 such as the X-ray protection signal (XRP). If a DC fault is detected in the power amplifier of one of the audio signal channel processors 32 , 34 , the power supply 44 is switched off and the loudspeaker 40 or 42 concerned is protected. These aspects of the receiver shown in FIG. 1 are explained in more detail with reference to FIGS. 2 and 3.

Fig. 2 shows a more detailed block diagram of parts of the television receiver shown in Fig. 1, which are particularly important in the present invention. Only the arrangement for one of the audio signal processors 32 , 34 is shown and described; a corresponding arrangement is used in the other processor. Power supply 44 supplies left and right stereo power amplifiers 48 , 50 via corresponding lines 46 b and 46 a with plus and minus power supply voltages with respect to ground. As said in connection with the audio power amplifiers, power supply 44 is a representative power supply and can, for. B. the main power supply or a secondary power supply derived from the main power supply or from the line end transformer or the like. Power amplifiers 48 , 50 have complementary output transistors which are connected in series via the plus and minus power supply voltages to corresponding loudspeakers 40 , 42 which are connected between corresponding connections of the output transistors and ground. This is shown in FIG. 3 and is discussed in more detail below. The loudspeaker 40 , 42 connected signal is also connected to Fehlerde tector 52 , which includes a low-pass filter 54 and egg NEN threshold detector 56 . A normal, loudspeaker 40 , 42 present audio signal is attenuated in the low-pass filter 54 , so that the detector is not "triggered" by this. However, a constant direct current passes through the filter and if the constant direct current is above or below a predetermined threshold, the detector 56 is actuated. The output signal from detector 56 is connected to a transistor 58 , so that the collector electrode of transistor 58 becomes "low" when an output signal from detector 56 is applied to the base electrode.

Transistor 60 symbolically represents the output of the X-ray radiation protection circuit 25 of the deflection and high-voltage part 23 . In response to a parameter indicating the possible generation of excessive x-rays, transistor 60 is made conductive. The "low" becoming the collector of transistor 60 causes the corresponding de signal input from microprocessor 62 to be "low". As already stated, the microprocessor 62 in the exemplary embodiment is programmed to switch off the power supply 44 . As shown in Fig. 2, the collector electrode of transistor 58 is connected to the input signal line of microprocessor 62 , so that a "low" signal from transistor 58 is responded as if an X-ray generating error has been detected, and Microprocessor 62 is operated so that it carries out its programmed measure to protect against the generation of excessive X-rays, for. B. Power supply 44 "turns off". As a result, the power supply voltages + Vb and -Vb are removed from power amplifiers 48 , 50 .

Fig. 3 shows a detailed schematic of error detector 52. Complementary audio power output transistors 64 (NPN) and 66 (PNP) of power amplifiers 48 , 50 are interconnected with their emitter electrodes and with their collector electrodes at power supply connection 46 b with positive voltage (+ Vb) or at power supply connection 46 a with negative Voltage (-Vb) connected. Power amplifiers 48 , 50 may be included in an integrated circuit, such as a uPC1188H, available from NEC, Japan. Alternatively, discrete devices can be used. When using discrete output transistors, it is not uncommon for thermally stabilizing components, such as resistors, to be inserted into the emitter circuits of the discrete devices.

Speaker 40 , 42 is connected between connection 68 of the emitter electrodes and ground. The "steady-state" voltage generated at connection 68 is a voltage between the plus supply voltage + Vb (e.g. +12 volts) and the minus supply voltage -Vb (e.g. -12 volts). Ideally, when transistors 64 , 66 match properly and are provided with voltage, terminal 68 has a no-load voltage of zero volts with respect to ground and no direct current flows through loudspeakers 40 , 42 . Transistors 64 and 66, however, rarely serve perfectly in their dynamic range or are ideally supplied with bias to their base electrodes, so that a certain, but relatively low, direct current often flows through the loudspeakers 40 , 42 .

Fault detector 52 has two input terminals 70 , 72 , one for each channel. The combination of the signal and the DC voltage present on each of the input lines 70 , 72 is connected to a low-pass filter 74 or 76 , the resistors 78 and 80 and capacitors 82 and 84 contains. In the exemplary embodiment, resistors 78 , 80 are each 220 KOhm and capacitors 82 , 84 are each 4.7 microFarads, so that the crossover frequency is clearly below 20 Hz. Connection 83 between resistor 78 and capacitor 82 and connection 85 between resistor 80 and capacitor 84 are connected through resistors 79 and 81 (also if 220 KOhm) to power supply terminal 46 b, which are selected so that connections 83 and 85 approximately + Vb / 2 are above ground, +6 volts above ground in the exemplary embodiment. The resistance of 220 KOhm of the resistors 78 , 80 is significantly higher than the resistance of about 8 ohms of the loudspeakers 40 , 42 , so that any bias current that is switched back from connection 83 , 85 to loudspeakers 40 , 42 is negligible. Resistors 71 , 73 short speakers 40 and 42 to ground and cause ken in the absence of speakers 40 , 42 a direct current return to ground for their corresponding input terminals 70 , 72nd The resistance values of resistors 71 , 73 are much higher than the internal DC resistance of speakers 40 , 42 , e.g. B. 3.2 ohm DC resistance for a speaker with 8 ohm nominal resistance, and therefore do not change the operation of the circuit.

The low pass filtered signal from input terminals 70 , 72 is connected to the base electrodes of complementary pairs of transistors 86 , 90 and 88 , 92 , respectively, with transistors 86 , 90 paired for one channel and transistors 88 , 92 paired for the other channel. The emitter electrodes of Transistor interfere 86, 88, 90 and 92 are all connected together with of a common terminal 94 which connected through between the plus power supply power supply voltage + Vb is applied to terminal 46 b and ground voltage divider resistors 96, 98 is +6 volts (same voltage like connections 83 , 85 ). The base electrodes of NPN transistors 86 , 88 and PNP transistors 90 , 92 are thus through voltage dividers 79 , 78 and 81 , 80 to +6 volts, and the emitter electrodes are through resistors 96 , 98 to +6 volts. If the voltage at one of the connections 83 , 85 rises above the Vbe (0.7 volt) of NPN transistors 86 , 88 , the line threshold of the transistors 86 and 88 is exceeded and this transistor becomes conductive. If the voltage at one of the connections 83 , 85 drops by the Vbe (0.7 volt) of PNP transistors 90 , 92 , the line threshold of the corresponding transistor is exceeded in a corresponding manner and the transistor in question becomes conductive. However, since resistors 78 and 79 and also resistors 80 and 81 form a 2: 1 voltage divider for the DC voltage at terminals 70 and 72 , the corresponding transistors only begin to conduct in the corresponding direction when there is a change of 1.4 volts.

Resistors 78 and 79 in combination with capacitor 82 , and resistors 80 and 81 in combination with capacitor 84 be a charging time constant for the occurrence of a DC voltage occurring at connections 70 , 72 at connections 83 and 85 , which causes in the manner discussed above that the transi connected to the corresponding connection are conductive. This is desirable so that the circuit does not respond to short-term DC peaks and thereby incorrectly indicates an error.

The collector electrodes of NPN transistors 86 , 88 of the channels 32 , 34 are connected to each other at the connection 100 , which is connected through resistor 102 to the plus power supply voltage line 46 b. PNP transistors 90 , 92 of the two channels are connected in a corresponding manner with their collector electrodes to one another at connection 104 and connected to ground via resistor 106 . The signal on connection 100 is connected through resistor 110 to the base electrode of transistor 108 . The emitter electrode of transistor 108 is connected to terminal 46 b with the power supply voltage + Vb, and the collector electrode is connected through resistor 112 to connection 104 .

As the voltage on connection 100 becomes lower (less positive) when the NPN transistors 86 , 88 on connections 70 and 72, respectively, detect an error with a positive effect, and the voltage on connection 104 becomes higher (more positive) when the PNP -Transistors 90 , 92 on connections 70 and 72 detect a fault with a negative effect, the phase-inverting PNP transistor 108 is present in order to change the voltage change of NPN transistors 86 , 88 indicating the fault, so that all fault displays for both Detector parts are the same. In this way, all fault-indicating voltages on connections 70 , 72 , whether plus or minus, lead to a voltage increase on connection 104 .

The x-ray protection circuit of the television receiver in Fig. 2 is designed to go "low" when an error is detected. Accordingly, the output of the detector shown in FIG. 3 should, for reasons of compatibility, also become "low" when an error is detected. Since the voltage on connection 104 goes "high" when an error is detected, phase inverter transistor 58 is used. The error-indicating voltage change at connection 104 is connected through a resistor 116 to a base electrode of transistor 58 , and an output signal detected in the audio system, indicating an error, is at the collector electrode of transistor 58 via a network with series resistor 120 and shunt resistor 122 with bus 118 ( Fig. 2) connected.

As already stated, in the exemplary embodiment, the ratio of the divider resistors 78 and 79 and resistors 80 , 81 is the same or corresponds to a ratio of 2: 1, so that half of the plus power supply voltage at connections 83 , 85 occurs. The voltage at connection 94 supplied by divider resistors 96 , 98 thus corresponds to the voltage at connections 83 and 85 . This correspondence between the two resistance divider voltages can be used for setting the sensitivity, or the magnitude of the DC voltage at connections 70 , 72 required to indicate an error. For example, if the divider ratio were chosen so that resistor 78 had twice the value of resistor 79 , each error change voltage at terminal 70 would be divided by a third and 2.1 volts DC would be necessary across speaker 40 to make a change of 0 To generate 7 volts on connection 83 . However, the open circuit voltage at connection 83 would then be two thirds of the 12 volt + Vb, or 8.0 volt. In such a case, the voltage at connection 94 would also be set to 8.0 volts.

If the voltages at connections 83 , 85 and 94 are the same, the plus and minus error voltages at loudspeakers 40 , 42 are symmetrical in every direction and the full Vbe must be developed in each direction so that the corresponding transistors become conductive and recognize an error. However, if the voltage at connection 94 were less than the voltage at connection 83 , which would provide part of the threshold voltage, a lower voltage increase at connection 83 would be required to lead to a detection. In such a case, of course, the threshold voltage for the PNP transistors would be increased and the symmetry would be lost. However, the threshold symmetry for plus and minus error voltages can be obtained by connecting diodes or zener diodes polarized in the forward direction in series with the emitter electrodes of transistors 86 , 88 , 90 , 92 . The threshold voltage required for detection can be changed in a corresponding manner. This change can be coordinated with a change in the ratio of the resistive dividers 78 , 79 and 80 , 82 .

Claims (13)

1. TV system with a video part ( 22 , 23 , 30 ) for preparing a video signal, an audio part ( 26 , 32 , 34 ) for processing an audio signal and at least one power supply unit ( 44 ) which supplies operating voltages ( 46 ) to the video ( 22 , 23 , 30 ) and the audio part ( 26 , 32 , 34 ), with:
Means ( 26 , 32 , 34 , 48 , 50 ) for amplifying the audio signal and for controlling a loudspeaker contained in the audio part,
first means ( 25 ) for detecting an error in the video part,
Means ( 60 , 62 , 63 ) for turning off at least a portion of the video portion in response to the detection of an error in the video portion, characterized by
second means ( 52 ) for detecting an error in the gain means, and
Means ( 58 ) for actuating the means for switching off ( 62 , 63 ) in response to the detection of an error by the second detection means. 2. System according to claim 1, characterized in that the video part contains means ( 23 ) for generating a high voltage for egg ne picture tube and the first detection means ( 25 ) are connected to the high voltage generating means. 3. System according to claim 2, characterized in that the first detection means ( 25 ) contain means for recognizing the conditions that correspond to the generation of excessive X-rays. 4. System according to claim 2, characterized in that the means for switching off contain a controller ( 62 ) which, when the video system error is detected, causes one of the power supply units ( 44 ) to be switched "off", and then acts the power supply is switched "on" a predetermined number of times to test whether the fault is still present and, if it is still present, stops trying to switch "on" again. 5. System according to claim 2, characterized in that the second means for detection ( 52 ) contain a low-pass filter ( 54 ) and a threshold detector ( 56 ). 6. System according to claim 2, characterized in that the second means for detecting a ver connected to the loudspeaker direct current detector ( 54 ). 7. System according to claim 1, characterized in that the audio part contains a plurality of channels, the amplification means ( 26 , 32 , 34 , 48 , 50 ) contain a plurality of amplifiers, one for each of the channels, and the second Detection means ( 52 ) comprise a plurality of detectors, one for each of the channels, each of the plurality of detectors having an input connected to the corresponding channel and having an output and the outputs being connected to the actuating means. 8. TV system with a video part ( 22 , 23 , 30 ) for preparing a video signal, an audio part ( 26 , 32 , 34 ) for processing an audio signal and at least one power supply unit ( 44 ) which supplies operating voltages to the video and audio delivers, with:
Means ( 26 , 32 , 34 , 48 , 50 ) for amplifying the audio signal and for controlling a loudspeaker contained in the audio part,
first means ( 25 ) for detecting an error in the video part,
Means for switching off ( 60 , 62 , 63 ) at least a part of the video part in response to the detection of an error in the video part, the means for switching off comprising x-ray protection means ( 25 , 60 ) which are connected to a microprocessor ( 62 ) are connected, which in turn is connected to at least one of the network parts ( 44 ) of the receiver, the microprocessor ( 62 ) being programmed such that it detects the receiver a predetermined number of times "on" when a video system error is detected. switches and tests whether the error is still present and, if it still exists, stops trying to switch the receiver "on", marked by
second means ( 52 ) for detecting an error in the means ( 26 , 32 , 34 , 48 , 50 ) for amplifying, and
Means for actuating ( 58 ) the means for switching off ( 62 , 63 ) in response to detection of an error in the second means for detection ( 52 ). 9. Detection system for detecting an error in one of a plurality of audio channels, with:
a plurality of sensor devices ( 52 ), each of which has a single input connected to a corresponding audio channel - for detecting a fault in the corresponding audio channel - and an output ( 118 ) at which an output signal indicating the fault is generated, characterized by power supply means ( 44 ) which supply operating current to each of the audio channels and
Means ( 62 , 63 ) for shutting down the power supply means in response to an output signal ( 118 ) indicative of an error detected by one of the sensor devices, the output signals of the plurality of sensor devices being connected in parallel to one another with the means for shutting down. 10. TV system with a video part ( 22 , 23 , 30 ) for preparing a video signal, an audio part ( 26 , 32 , 34 ) for processing an audio signal and at least one network part ( 44 ), which operating voltages to the video and the audio part delivers, characterized by
Means ( 26 , 32 , 34 , 48 , 50 ) for amplifying the audio signal and for controlling a loudspeaker contained in the audio part, the means comprising a low-pass filter ( 54 ) and threshold voltage detection means ( 56 ),
first means ( 25 , 60 ) for recognizing an error in the video part,
means ( 62 , 63 ) connected to at least one of the power supply units for switching off at least part of the video part in response to the detection of an error in the video part,
second means ( 54 , 56 ) for detecting an error in the amplifying means, and
Means ( 58 ) for actuating the means for switching off ( 62 ) in response to the detection of an error in the second detection means ( 54 , 56 ). 11. The device according to claim 10, characterized in that the second means ( 54 , 56 ) for detecting contain means for setting the threshold at which an error he is recognized. 12. The apparatus according to claim 10, characterized in that the means for setting the threshold at which an error is detected are contained in the low-pass filter ( 54 ). 13. TV system with a video part ( 22 , 23 , 30 ) for preparing a video signal, an audio part ( 26 , 32 , 34 ) for processing an audio signal and at least one network part ( 44 ), which operating voltages to the video and the audio part delivers, with:
Means for amplifying ( 26 , 32 , 34 , 48 , 50 ) the audio signal and for controlling a loudspeaker included in the audio part,
first means ( 25 ) for recognizing an error in the video,
with at least one of the power supply units ( 44 ) connected with means (62, 63) for switching off at least part of the video part ( 22 , 23 , 30 ) in response to the detection of an error in the video part, characterized by
second means ( 52 ) for detecting an error in the amplifying means, the second means for detecting ( 52 ) a low pass filter ( 54 ) having a series resistor ( 78 , 80 ) and a parallel capacitor ( 82 , 84 ), threshold voltage means with biasing means ( 81 , 79 ), the series resistor ( 79 , 80 ) and a semiconductor device ( 88 ) with a line threshold, wherein the series resistor ( 78 , 80 ), the parallel capacitor ( 82 , 84 ) and the biasing means ( 81 , 79 ) determine the length of time of the predetermined period of time, a voltage ratio of the series resistor ( 78 , 80 ) and the biasing means ( 81 , 79 ) representing means for adjusting the sensitivity to the threshold of the error that is detected, and
Means ( 58 ) for actuating the means for switching off in response to the detection of an error by the second detection means.